The present invention pertains to a communication circuit for a multiprocessor system for multitasking applications. The system provides that independent tasks of different jobs or related tasks of a single job may be run on multiple processors. Typical inter-CPU communication in prior art machines is accomplished throughout the main memory, in a "loosely coupled" manner. Inter-CPU communication of this nature is hampered by the need to repetitively resort to relatively slow main or central memory references, and by access conflicts between the processors. The multiprocessor communication system of the present invention overcomes the substantial delays and software coordination problems associated with loosely coupled multiprocessing by providing a "tight-coupling" communications circuit between the CPUs which is independent of the shared or central memory.
The tight-coupling communication circuits of the present invention provide a set of shared registers which may be accessed by either CPU at rates commensurate with intra-CPU operation. Thus, the shared registers provide a fast inter-CPU communication path to minimize overhead for multitasking of small tasks with frequent data interchange. The present multiprocessor system also provides that tasks may be coupled through the shared memory as provided by the prior art. However, the tight-coupling communications circuits provide a hardware synchronization device through which loosely coupled tasks as well as tightly-coupled tasks may be coordinated efficiently.
An example of a tight-coupling communication scheme is disclosed in U.S. Pat. No. 4,636,942. While that scheme is highly effective for a system in which the processors may be closely packed together, it was not designed with the alternate possibility in mind. The present invention is designed particularly for use in multiprocessor systems wherein the processors are physically separated by necessity such that common communication resources cannot be accessed by all processors without significant signal propagation delays.